I don’t have a mac to test it on but maybe nuke the urbackup client and reinstall? You probably already tried that.
Checking a Linux client, /usr/local/var/urbackup/pw_change.txt shows that it is owned by root with rwx (700) permissions. That file contains a key on a single line.
This post shows a similar length key in the pw_change.txt file which was a permissions issue although it was on the server.
Good luck!